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Common Stock Incentive Plan - Restricted Stock (Details) - Restricted Stock - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Common Stock Incentive Plan      
Remaining unamortized share-based compensation expense $ 18.0    
Number of shares      
Outstanding nonvested shares, beginning of year 513,523 456,282 527,176
Shares granted 149,264 260,171 161,949
Shares vested (183,381) (200,066) (205,248)
Shares forfeited (3,638) (2,864) (27,595)
Outstanding nonvested shares, end of each period 475,768 513,523 456,282
Weighted average price      
Outstanding nonvested shares, beginning of year (in dollars per share) $ 48.33 $ 30.46 $ 29.02
Shares granted (in dollars per share) 59.21 54.14 50.87
Shares vested (in dollars per share) 46.65 43.26 37.70
Shares forfeited (in dollars per share) 56.57 48.15 45.58
Outstanding nonvested shares, end of each period (in dollars per share) $ 52.32 $ 48.33 $ 30.46
Service-based vesting | Directors      
Number of shares      
Shares granted 28,000    
Service-based vesting | Employees      
Number of shares      
Shares granted 121,264    
Three year vesting period | Service-based vesting | Directors      
Common Stock Incentive Plan      
Vesting period 3 years    
Number of shares      
Shares granted 8,000    
Three year vesting period | Service-based vesting | Directors | Maximum      
Common Stock Incentive Plan      
Years of service 6 years    
Vesting rights percentage 33.33%    
Two year vesting period | Service-based vesting | Directors      
Common Stock Incentive Plan      
Years of service 6 years    
Vesting rights percentage 50.00%    
One year vesting period | Service-based vesting | Directors      
Common Stock Incentive Plan      
Years of service 7 years    
Vesting rights percentage 100.00%    
Immediate vesting | Service-based vesting | Directors      
Number of shares      
Shares granted 20,000    
Immediate vesting | Service-based vesting | Directors | Minimum      
Common Stock Incentive Plan      
Years of service 8 years    
Five year vesting period | Service-based vesting | Employees      
Common Stock Incentive Plan      
Vesting period 5 years    
Number of shares      
Shares granted 48,338    
Four year vesting period | Service-based vesting | Employees      
Common Stock Incentive Plan      
Vesting period 4 years    
Number of shares      
Shares granted 72,926    
2012 Plan | Maximum      
Common Stock Incentive Plan      
Vesting period 5 years 5 years 5 years